A conventional solar thermal electric power generation system 101 shown in FIG. 4 has a configuration wherein: sunlight is collected by means of a concentrating type heat collecting unit (hereinafter will be referred to as “heat collecting unit”, simply) 102; a heating medium absorbs collected sunlight as thermal energy; and the heating medium is supplied to a heat exchanging device 103 in order to generate steam by utilizing heat of the heating medium. Saturated steam generated by the heat exchanging device 103 is then superheated by means of a superheater 104. A steam turbine 105 is driven by such superheated steam to generate electric power. In the figure, reference characters 106 and 107 denote an electricity generator and a condenser, respectively.
Methods of collecting incidental solar radiation are roughly classified into the central tower type and the parabolic trough type, the desired one of which is selected in view of various conditions of an installation area and a purpose of use. In many cases, a distribution type of heat collecting unit is employed. The heat collecting unit 102 of the parabolic trough type uses trough-shaped reflectors 102a having a parabolic section in an X-Y plane and configured to reflect sunlight thereon in order to collect it on its focal point. Heat absorbing tubes 108 each extending through the focal points of reflectors 102a along the Z-axis allow a heating medium to pass therethrough in order to collect solar heat. The heat absorbing tubes 108 and heating medium supply piping 109 connected thereto allow the heating medium to circulate between a heat exchanging device and the heat collecting unit. A special oil is generally used as the heating medium. The heating medium absorbs, for example, solar heat to reach a high-temperature condition of about 395° C., releases heat in order to generate steam in the heat exchanging device 103 thereby assuming a low-temperature condition of about 295° C., and returns to the heat collecting unit 102. Reference character 110 denotes a heat storage system.
As can be seen from FIG. 5, a plot of the solar energy density variance during one day, the conventional solar thermal electric power generation system 101 can operate only during day time from sunrise to sunset. For this reason, the operation of the system is stopped at night and must be restarted the next morning. FIG. 5 plots the solar energy density variance during one day at a region in North Africa. Curves plotting average energy densities in July and December are shown respectively in FIG. 5, and curves plotting average energy density variance in other months are considered to fall within the range between the two curves shown. As shown, the intensity of solar thermal energy reaching the heat collecting unit 102 varies from zero to maximum during one day. Therefore, electric power generation cannot continue day and night.
In an attempt to solve this problem, an integrated solar combined cycle electric power generation system has been proposed which combines the above-described steam turbine electric power generation system only relying upon solar heat with the gas turbine combined cycle electric power generation (see European Patent Laid-Open Publications Nos. 0750730 and 0526816, for example). Such a new concept of solar thermal electric power generation system is intended to generate electric power even during nighttime or cloudy days during which solar heat cannot be utilized, by combination of the gas turbine electric power generation with the steam turbine electric power generation by utilizing steam generated in a waste heat recovery boiler. The system thus configured can be expected to continue electric power generation day and night. Such an integrated solar combined cycle electric power generation system includes a heat collecting unit configured to generate saturated steam directly from water and supply it to a steam turbine without using a special heating medium or a heat exchanging device. The electric power generation system according to European Patent Laid-Open Publication No. 0750730 is configured to mix the saturated steam generated in the heat collecting unit with steam generated from a high-pressure turbine for superheating the saturated steam before supplying it to the steam turbine. On the other hand, another kind of the electric power generation system according to European Patent Laid-Open Publication No. 0526816 is configured to mix the saturated steam with the steam generated from a high-pressure turbine and then superheat the saturated steam by means of a reheater of the waste heat recovery boiler before supplying it to the steam turbine.
Irrespective of whether or not the solar thermal electric power generation system is the integrated solar combined cycle electric power generation system, there exists an unavoidable problem. This problem is associated with the condition of solar radiation onto the surface of the Earth incidentally changing and fluctuating (with time) during daytime. In the solar heat collecting unit, heat transfer from solar heat to steam or other heating medium is mostly based on solar radiation conditions. Accordingly, the temperature of steam or other heating medium absorbing solar heat fluctuates in exact response to change in the condition of sunshine onto the surface of the Earth as a natural phenomenon. Since such a fluctuation occurs according to nature, it is difficult to predict accurately the time at which the fluctuation occurs and the degree of the fluctuation. As a result, the heating medium to be supplied to the heat exchanging device 103 in the solar thermal electric power generation system shown in FIG. 4 fluctuates in its condition including its temperature and the like, thus repeatedly imposing thermal stress on the heat exchanging device 103. With the integrated solar combined cycle electric power generation system according to the European Patent Laid-Open Publication No. 0750730, the condition of steam to be supplied to the steam turbine (including its temperature, pressure, wetness and dryness and the like) fluctuates, which may cause various problems with the steam turbine. With the integrated solar combined cycle electric power generation system according to the European Patent Laid-Open Publication No. 0526816, steam to be supplied to the waste heat recovery boiler and the like fluctuates in its condition, which will result in thermal stress repeatedly imposed on the waste heat recovery boiler.
Such a fluctuation in sunshine condition, which is caused by, for example, clouds, sandstorms or a like factor, causes the amount of collected heat to decrease rapidly and largely. Further, when each of the aforementioned reflectors is bent by wind incidentally, sunlight cannot be sufficiently concentrated on the aforementioned heat absorbing tubes. This may also cause a fall of the temperature of the heating medium or the like.